1. Field of the Invention
The present invention relates to a technology for reducing and stabilizing a beam spot size on a scanning surface in an optical scanning device.
2. Description of the Related Art
Recently, improvement in image quality of images output by “image forming apparatuses, such as multifunction peripherals (MFPs)” through optical scanning is being more and more demanded. To meet such a demand, a beam spot size at the time of optical scanning needs to be reduced and stabilized. “The stabilization of the beam spot size” can be attained by expanding a depth allowance of a light beam (a defocusing distance in an optical axis direction, which is determined such that a beam spot size is maintained within an allowable range). Regarding the depth allowance, it is widely known that the following relationship is satisfied.d∝w2/λwhere d is depth allowance, w is beam spot size, and λ is operation wavelength for optical scanning. In other words, if the depth allowance increases, the beam spot size increases in proportion to the depth allowance. Therefore, it has been difficult to attain both reduction and stabilization of the beam spot size at the same time.
One approach to “expand a depth allowance” and maintain a small beam spot size at the same time may be to use a Bessel beam.
A “Bessel beam” is disclosed in, for example, Japanese Patent No. 3507244. Specifically, it is disclosed that the Bessel beam has a side-lobe with an extremely high light intensity and a high-order side-lobe with a relatively high light intensity. Due to such characteristics, if the Bessel beam is used for optical scanning, light use efficiency is lowered, which makes it difficult to increase a processing speed of an image forming operation to the recently-required level.
The inventors of the present invention examined a beam spot image formed by using an optical system of an optical scanning device based on experiments using wave optics. As a result, the inventors found that it is possible to suppress an increase in a beam spot size (widening of a main lobe) even when a focusing position of a beam spot is shifted from a target position on a scanning surface and to expand a depth allowance by performing a “phase adjustment operation” in the following manner. That is, the “phase adjustment operation” is performed for adjusting a phase of a wave front of a light beam incident on the scanning surface so that the peak intensity of a side-lobe (a side-lobe adjacent to a main lobe) in a beam profile of a focusing position of a scanning lens is “slightly increased” to the level that does not degrade the quality of an image to be output.
Meanwhile, “a divergence angle of a laser beam” emitted from a typical semiconductor laser (e.g., an edge-emitting laser and a surface emitting laser) used as a light source of an optical scanning device generally depends on each product. If such a variation is not considered when installing a semiconductor laser as a light source, “effective fluctuation in a numeric aperture (NA) of an optical system in the optical scanning device” occurs, resulting in causing fluctuation in a beam spot size on a scanning surface and increasing the beam spot size.
To prevent the above-mentioned problems, the conventional optical scanning device has been configured to control “a width of a light beam to be incident on a focusing surface” by using an aperture after the light beam is emitted from a semiconductor laser. For example, Japanese Patent No. 2685252 discloses conventional apertures having an elliptical shape and a circular shape.
The inventors found that there is a correlation between the effect of “expansion of a depth allowance by a phase adjustment operation” and the shape of an aperture opening. In other words, to what degree a depth allowance can be expanded by a phase adjustment operation depends on the shape of the aperture opening.
More particularly, the inventors found that an aperture opening having “a rectangular shape”, which is a typical shape in the conventional technology, “is not sufficiently effective” for expanding a depth allowance by a phase adjustment operation. This is because side-lobes obtained by using a rectangular aperture opening are localized in the main-scanning direction and in the sub-scanning direction with relatively high peak intensities. Therefore, a beam spot size (a diameter of a light beam including the side-lobes and determined by using 1/e2 as a threshold) is deformed.
On the other hand, if an elliptical aperture opening is used, side-lobes are distributed around a main lobe. However, a depth allowance can be expanded by a phase adjustment operation mainly due to side-lobes in the main-scanning direction and in the sub-scanning direction. Therefore, if “the side-lobes are distributed around the main lobe”, “side-lobes that are less contributive to expansion of a depth allowance (i.e., side-lobes neither in the main-scanning direction nor in the sub-scanning direction)” occur with relatively high light intensities. As a result, a depth allowance cannot be effectively expanded.
Another conventional aperture having an aperture opening formed into a shape “other than an elliptical shape and a rectangular shape” is disclosed in, for example, Japanese Patent No. 2685252, Japanese Patent Application Laid-open No. 2005-266258, and Japanese Patent Application Laid-open No. 2002-006247. However, a correlation between the shape of the aperture opening and the effect of “the expansion of a depth allowance by a phase adjustment operation” is not disclosed in the above Patent Literatures. In other words, the inventors has newly found that “a depth allowance can be expanded by a phase adjustment operation” and there is a correlation between the shape of the aperture opening and the effect of “the expansion of a depth allowance by the phase adjustment operation”, which will be described in detail later.